This invention relates to composite structures, and more particularly to composite airfoils.
In aircraft applications, weight reduction of aircraft components is highly desirable, but can be difficult to achieve without sacrificing strength, safety and durability, especially with rotating hardware. Rotary components such as propeller blades, fan blades, rotor blades, and other propulsory blades or airfoils are subject to high loads from centrifugal forces, steady and vibratory bending loads, and impact loads from foreign objects. Propulsory blades constructed of composite materials can exhibit acceptable levels of tensile strength and other material properties and are typically lighter than their wood and metal counterparts. Many composite materials thus improve structural efficiency and allow thinner blades and improved aerodynamic performance. A typical propulsory blade may include a shell and an internal load-bearing spar that is connected to the shell through adhesive bonding at an attachment region. The shell is typically constructed of lightweight composite material, such as fiber-reinforced resin, and the spar is typically constructed of a metal or composite material and bonded to the interior of the shell. The spar extends from within the shell cavity and terminates in a root portion that is adapted for connection to a rotor shaft of an aircraft engine or the like. The root portion is also typically constructed of a metallic material.
However, the attachment region between the composite shell and the spar and/or root portion is relatively heavy and structurally inefficient, since the attachment region is designed such that loads on the shell are transferred to the root portion through shear forces. Inter-laminar shear forces in the shell are first transferred from one high strength composite layer to another, and then to the spar and/or root portion through shear forces at the attachment region. These shear forces are relatively weak when compared to the tensile strength of the composite layers. Thus, current propulsory blade construction does not take advantage of the higher tensile forces typically found in the composite layers.
The design of a connection joint between the composite shell and root portion that achieves acceptable uniform stress levels is an inexact science. Areas at the edges of the joint can develop high peel stresses, which greatly reduce shear strength and can lead to progressive delamination between composite layers, in the composite-to-adhesive joint, within the adhesive itself, as well as in the adhesive-to-metal joint, requiring monitoring and costly inspections and/or separate xe2x80x9cbackupxe2x80x9d joints to preclude premature blade failure. If a peeling force is present, the relatively low resistance to shear force can drop by a factor of two or more. Accordingly, adhesive joints typically require large, heavy areas of carefully tapered composite and metal surfaces to reduce stresses.
Thus, there is a continuous need to develop a practical and efficient composite propulsory blade that is safely and securely joined to a root portion.
According to the invention, a composite blade assembly comprises a root portion and blade portion that is connected to the root portion. The root portion comprises inner and outer rings and the blade portion comprises at least one layer of composite material that is folded to form a loop. The inner ring is disposed in the loop and the outer ring is disposed outside the loop such that a portion of the loop is positioned between the inner and outer rings. In this manner, the loop portion is held in compression between the inner and outer rings at least when the composite blade assembly is subject to centrifugal force to thereby prevent separation of the root portion and blade portion.
Further according to the invention, a composite blade assembly comprises a root portion and a blade portion. The root portion has an inner ring and an outer ring. The blade portion has a plurality of inner and outer layers of composite material that are folded to form a loop. The inner ring is disposed in the loop and the outer ring is disposed outside the loop such that a portion of the loop is positioned between the inner and outer rings. The blade portion forms a hollow interior. A core member is located within the hollow interior adjacent the layers of composite material. A base is positioned against a lower surface of the loop, with the loop and inner ring being sandwiched between the outer ring and the base.